需要用到的第三方库：
permission_handler: ^8.1.3 //权限请求
image_picker: ^0.8.0 //图片选择
crop_your_image: ^0.6.0+1 //图片裁剪

代码：

//打开相册选择图片
  chooserPic() async {
    final ImagePicker _picker = ImagePicker();
    final XFile? image = await _picker.pickImage(source: ImageSource.gallery);
    print("ChooserImage:${image!.path}");
    Uint8List imageByte = await image.readAsBytes();
    PageUtil().open(context, CropImage(imageByte));
  }

PageUtil().open()是我自己写的路由跳转方法，读者可以自行替换成下面的常规写法

  void open(BuildContext context, Widget page) {
     Navigator.push(context, MaterialPageRoute(builder: (context) => page));
  }

CropImage页面：

class CropImage extends StatefulWidget {
  Uint8List imageByte;

  CropImage(this.imageByte);

  @override
  State<StatefulWidget> createState() {
    return CropImageState();
  }
}

class CropImageState extends State<CropImage> {
  final _controller = CropController();

  CropImage() {  //确认裁剪时调用这个方法
    _controller.crop();
  }

  //将回调拿到的Uint8List格式的图片转换为File格式
  SaveImage(Uint8List imageByte) async { 
    EasyLoading.show();
    final tempDir = await getTemporaryDirectory();
    final file = await new File('${tempDir.path}/image.jpg').create();
    file.writeAsBytesSync(imageByte);
    print("${file.path}");
    EasyLoading.dismiss();
  }

  @override
  Widget build(BuildContext context) {
    return MyPage( //MyPage是笔者自己写的组件，读者可以忽略，不是主要代码
        "頭貼裁切",
        Column(
          children: [
            Container(height: 24),
            Expanded(  //Crop是裁剪控件
                child: Crop(
                  image: widget.imageByte,
                  controller: _controller,
                  onCropped: (image) { //裁剪完成的回调
                    SaveImage(image);
                  },
                  aspectRatio: 1,
                  initialSize: 0.8,
                  withCircleUi: true,
                  baseColor: Colors.black,
                  maskColor: Colors.black.withAlpha(150),
                  cornerDotBuilder: (size, edgeAlignment) => const DotControl(color: Colors.white54),
                )),
            Container(
                padding: EdgeInsets.fromLTRB(35, 16, 35, 16),
                child: Row(mainAxisAlignment: MainAxisAlignment.spaceBetween, children: [
                  Text("符合頭貼尺寸的裁切範圍", style: TextStyle(fontSize: 12, color: MyColor.White)),
                  Button("儲存", radius: 13,
                      height: 26,
                      width: 71,
                      bgColor: MyColor.Red,
                      fontSize: 12,
                      onTap: CropImage) //点击开始裁剪
                ]))
          ],
        ),
        bgColor: MyColor.Black,
        isBlackTxt: false);
  }
}

主要代码：裁剪区域

            Crop(
                  image: widget.imageByte,
                  controller: _controller,
                  onCropped: (image) {//裁剪完成的回调
                    SaveImage(image);
                  },
                  aspectRatio: 1,
                  initialSize: 0.8,
                  withCircleUi: true,
                  baseColor: Colors.black,
                  maskColor: Colors.black.withAlpha(150),
                  cornerDotBuilder: (size, edgeAlignment) => const DotControl(color: Colors.white54),
                )